1. Field of the Invention
The present invention relates to a technique of guaranteeing the integrity of digital data.
2. Description of the Related Art
Digital cameras and the like are recently widespread, which record captured images in a recording medium not as conventional silver halide photos or 8-mm film but as digital image data. Insurance companies which handle photos of evidence in tasks concerning accidents or construction firms which handle the records of the progress at the sites of construction also think of using digital image data. However, digital image data is easy to alter using, e.g., a commercially available photo retouch tool. For this reason, digital image data is less reliable than conventional silver halide photos and therefore poor in capability as evidence. To solve this matter, a method as disclosed in patent reference 1 has been proposed. According to this reference, a digital camera holds secret information in advance. Upon capturing an image, the digital camera signs the digital image data using the secret information. After the image capturing, validation processing is performed using the generated signature information, thereby guaranteeing the integrity of the digital image data.
According to the technique of U.S. Pat. No. 5,499,294, if at least one bit of digital image data has been changed, it is determined by validation processing that “the image data has been altered”. Hence, even if the contents of digital image data have not substantially been changed (for example, when image processing such as enlargement/reduction or rotation has been performed), it is determined by validation processing that “the image data has been altered”.
To solve this matter, a method disclosed in Japanese Patent Laid-Open No. 2003-298579 has been proposed. According to this reference, signature information is generated based on the feature amount of captured image data, instead of directly generating it from digital image data. In this technique, if the feature amount after image processing is the same as that before image processing, it is determined by validation processing that “the image data has not been altered”. In this reference, to ensure tolerance of reduction processing of digital image data, the coefficient of a low frequency component in a wavelet transformation domain is used as the feature amount.
There is conventionally no suitable alteration validation technique of correctly determining that “image data has not been altered” when digital image data has undergone only rotation without any substantial change in contents.